The invention relates to a shaped roof element according to the preamble of claim 1 and particularly to a roof element for a roof covering, the shaped roof element being manufactured of a plate material, the roof element comprising a first and a second side edge, an upper edge and a lower edge, a profile substantially parallel with the side edges and a series of steps comprising two or more steps extending substantially parallel to the upper and the lower edges, the steps being produced so that the lower edge is one of the steps.
Steel sheet roofs are subject to cut edge corrosion, when precut roof elements joined together to overlap at their border areas become wet. Generally the reason for this is that the borders of the roof elements are not coated, whereby moisture is allowed to penetrate between the sheet and the coating and cause detachment of the coating. Water tends to penetrate between the overlapping roof elements due to capillary force, for example, and other properties of the seam areas, which significantly enhances edge corrosion. In the prior art, attempts have been made to prevent edge corrosion by painting or lacquering the edges of roof elements in different ways. Also seams running parallel to the eaves between successive roof elements in the direction of the roof pane have been formed with the view of preventing water from flowing between the roof elements in the seam area. When different roof elements comprising steps parallel to the eaves are used, the seam area is produced to the vertical portion of the roof element steps, where the wet period is significantly shorter than in seam areas parallel to the surface of the roof elements. In the prior art it has been possible to provide these vertical step portions with different drips by bending the edge of the topmost plate at the seam area away from the vertical portion for preventing water from running between the roof elements. An alternative known method is to provide these vertical portions with different round bends, in which the edge of the roof element is bent out of sight underneath the roof element.
A problem with the above prior art arrangements is that different ways of painting and lacquering the edges are not cost-effective from the manufacturing point of view because of the drying time needed for the paints and the lacquers. Moreover, the paints and lacquers may easily crack and become detached during the installation or use of the roof elements. Although in the prior art seams have been made to vertical portions of roof elements arranged stepwise in the direction of the roof pane, these seams, together with drips, if any, associated with them are not capable of solving the problem of cut edge corrosion these seams are subjected to, because despite its shape, the cut edge of the roof element edge parallel to the eaves is subjected to moisture from the environment. In addition, solutions employing different round bends, in which the edge of a roof element is bent underneath the roof element, create a problem of mechanical abrasion when overlapping roof elements move in relation to one another during installation and normal use. The edge or fold bent as a round end underneath the roof element thus rubs against the plate underneath, thereby damaging its surface and exposing the rubbed area to corrosion.